When the Senate left town for the August recess, senators confirmed nominees for an assortment of posts ranging from Amtrak board member to ambassador to the Vatican. But one member of Congress' military promotion was left in limbo.

Army Reserve Col. Joe Heck is up for what appears to be a routine promotion to be a brigadier general. But no seems to be able to say why Heck, who is known on the House floor as "the gentleman from Nevada," has been stopped in his tracks.

The GOP congressman's nomination is apparently being held up by something or someone, given that it has been languishing on the Senate calendar since May. The reasons for the delay could involve the constitutionality of promoting a lawmaker to the rank of general, or they could simply be related to Pentagon policy. Then there's the question of whether a member of Congress should also serve as a general overseeing all Army Reserve field medical units on the East Coast.

Regardless, there's no evidence of a hold originating from a Senate office.

Still, the delay likely has been caused by either the Senate or the Defense Department. But Senate leadership and Pentagon sources — both military and civilian — have professed to have no idea what prompted the delay in Heck's elevation or have played coy, leaving the mystery unresolved.

As is customary, the Senate Armed Services Committee declined to comment on the nomination, and the Pentagon wouldn't say much more.

"Rep. Heck was nominated to the position of brigadier general, but has not yet been confirmed by Congress. It is inappropriate to comment on pending nominations," said Lt. Cmdr. Nate Christensen, a Defense Department spokesman.

Though CQ Roll Call contacted numerous Pentagon and congressional sources in both parties, only theories of what might be going on were offered.

Some sources speculated that the holdup on Heck's nomination could be a broader policy issue, rather than a problem specifically with the Nevada congressman. The Pentagon could have asked the Senate Armed Services Committee to postpone a vote on Heck’s nomination until the issue is resolved, for example.

When Heck became a member of Congress, he would generally be expected to transfer to the Standby Reserve, which consists of reservists who are not required to train or be parts of units. Some members of the Standby Reserve are not eligible for promotion, although members of Congress can be on the Active Status List, meaning they can receive a boost in rank.

However, being a member of the Standby Reserve — and thus, not a particularly active servicemember — could make it difficult to receive a promotion from a colonel to a one-star general, considering the very small percentage of reserve officers who make it to the rank of brigadier general.

The second-term House member is an osteopathic physician by trade who has been an Army reservist since 1991. According to the Pentagon, Heck's confirmation would allow him to take over as deputy commander of the troop program unit at the Third Medical Command, which is based in Atlanta.

Heck's spokesman said his boss keeps his military business separate from the congressional office and the spokesman couldn't comment on whether he had transferred to the Standby Reserve after becoming a member of Congress. Heck's office also professed to not know why the promotion has been held up.

Adding to the mystery is the fact that Heck was rather swiftly approved by the Armed Services Committee and has been on the Senate calendar since shortly before Memorial Day.

There is precedent for sitting members to receive Reserve promotions. Sen. Lindsey Graham, R-S.C., was promoted to colonel in the Air Force Reserves in 2004. A former active duty member of the Judge Advocate General's Corps, Graham has been a reservist since 1995.

But one of Heck's potential issues might actually come out of a case involving Graham.

The U.S. Court of Appeals for the Armed Forces held in 2006 that Graham's service as a judge hearing cases in military court ran afoul of the Constitution's "incompatibility clause."

In the opinion, the court held that a member of Congress could not simultaneously fulfill the responsibilities of a military judge.

"A position that requires the exercise of those powers is an office of the United States and cannot be filled by a person who simultaneously serves as a Member of Congress," the court said. The court stressed that the case itself was decided rather narrowly, applicable to the rare circumstance in which a senator served as a judge.

"The present case does not require us to determine the qualification of an individual to serve as a Member of Congress; nor does it require us to define the scope of the standing of citizens in general to litigate the relationship between congressional service and membership in the Reserves," the judge wrote.

When a similar issue came before the Supreme Court in the 1970s case of Schlesinger v. Reservists Committee to Stop the War, the court avoided the underlying question of service itself.

"The Court resolved the case on procedural grounds, finding that the Reservists Committee did not have standing to raise the matter in court, and did not address the substantive constitutional claim. Other courts have dealt with related issues, including what positions constitute offices of the United States," a 2009 Congressional Research Service report explained.